Structural Concrete

Journal: Structural Concrete

Abbreviation

Struct. concr.

Publisher

Wiley

ISSN

1464-4177
1751-7648

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Publications 1 - 10 of 32

Digital Nervi: Performance-based design of lightweight isostatic ribbed slab with 3D-printed concrete formwork
Hua, Hao; Liu, Yizhuo; Hovestadt, Ludger; et al. (2025)
Structural Concrete
The material efficiency of cast-in-situ concrete floor slabs can be significantly improved through topology optimization and 3D-printed formwork. The isostatic ribbed slab (IRS), a legacy of Pier Luigi Nervi, aligns the ribs with the isostatic lines of the principal bending moments. This work introduces the Functional IRS as a cast-in-situ lightweight concrete slab system using 3D-printed permanent formwork. The Functional IRS serves as a compact functional ceiling plenum that integrates skylights and building devices such as air ducts. To create openings in ribs, the coupled pods (formwork) were developed to comply with the material-extrusion 3D concrete printing (3DCP) process. A 1:5 scale mockup with curved supporting walls was constructed to verify the feasibility of the construction method. With supports of irregular shape, optimized IRS saves about 30% of the material compared to grid slab. The lightweight 'digital Nervi' slab presents a holistic response to global low-carbon concerns by integrating computational optimization and additive manufacturing.
Large‐scale experiments on concrete hinges under general loading
Markic, Tomislav; Kaufmann, Walter (2023)
Structural Concrete
The structural behavior of concrete hinges under general loading is far from being properly understood, which is to a large extent due to the lack of pertinent experimental data. This paper contributes to filling this knowledge gap by presenting and discussing the results of an experimental campaign on one-way Freyssinet concrete hinges. Seven large-scale concrete hinges were tested in the Large Universal Shell Element Tester, which allowed the investigation of their behavior under general loading by all six stress resultants. A combination of digital image correlation and distributed fiber optic measurements allowed a deeper insight into the structural behavior of the specimens. The tested hinges could sustain very high axial stresses exceeding 4.5 times the uniaxial concrete compressive strength, large rotations of over 60 mrad, and shear stresses in transverse and longitudinal directions up to 2.24 times the axial compressive stress. The resistance to bending moments about the strong axis and torques also proved to be significant. A moderate amount of reinforcement crossing the hinge throat considerably increased the shear resistance at low axial stresses and produced a ductile shear behavior.
Load-bearing and local bond behavior of two-span concrete slab strips with hybrid steel/CFRP reinforcement
Näsbom, Andreas; Thoma, Karel; Reckinger, Nathalie; et al. (2025)
Structural Concrete
Utilizing brittle components in structural concrete challenges state-of-the-art verification procedures based on limit theorems of the theory of plasticity. Unresolved questions regarding deformation capacity, force redistribution, and the influence of generally unknown self-equilibrated stress states impede universally applicable verification frameworks for concrete structures without regions that can fully plasticize. A prerequisite for establishing such frameworks is a detailed understanding of the mechanical behavior, the interaction of brittle and ductile reinforcement components and the impact of self-equilibrated stress states. This paper investigates the structural behavior of two-span concrete slab strips with hybrid steel/CFRP reinforcement, that is, the combination of conventional ribbed steel reinforcing bars (ductile elements) with sand-coated CFRP rods (linear elastic, brittle elements). The study focuses on (i) the global load-bearing behavior, (ii) the influence of self-equilibrated stress states, and (iii) the local bond behavior, which are all discussed regarding initial (backbone) and cyclic loading, respectively. Statistical evaluations of nominal bond shear stress as a function of slip and reinforcement strains-all obtained from Distributed Fiber Optical Measurements-allowed for reliable quantification of the bond behavior and provided consistent insight despite considerable scatter. All data gathered in the presented tests are published open source together with a separate data repository guide.
Tension chord model for CFRP-prestressed structural concrete
Näsbom, Andreas; Merz, Paul; Thoma, Karel; et al. (2025)
Structural Concrete
This paper introduces a mechanical model for steel-reinforced concrete prestressed with bonded CFRP rods and presents the findings of an experimental campaign conducted to validate the model. The study is part of a project aiming at developing a railway bridge system in Switzerland that utilizes stainless steel reinforcing bars combined with pretensioned sand-coated CFRP rods to (i) maximize durability and (ii) achieve reasonable plastic deformation capacity despite the inherent brittleness of the prestressing material. The proposed Tension Chord Model for CFRP-prestressed Structural Concrete (TCM-cfrp) extends the established Tension Chord Model for Structural Concrete by introducing a constant, rigid-perfectly plastic bond shear stress-slip relationship between concrete and CFRP, allowing for computationally efficient closed-form solutions to determine the load-deformation behavior, crack widths, and reinforcement stresses. The validation against the experimental results reveals (i) satisfactory model predictions of the mean strains, the stresses at the crack and the crack widths and (ii) insensitivity of the structural behavior on local bond shear stress distributions, thus supporting the simplification of assuming constant bond shear stresses. The TCM-cfrp is capable of capturing all stress states in tension chords of railway bridge girders and other structural elements relevant for serviceability, ultimate, and to some extent, fatigue limit state verifications. The limitations of the model lie in (i) the simplified treatment of the crack formation process, overestimating deformations but underpredicting crack widths prior to stabilized cracking and (ii) its inability to realistically model local strain and stress distributions between the cracks, which is however of limited relevance for most structural engineering purposes.
Strip loading in reinforced concrete: Mechanical modeling and experimental validation
Morger, Fabian; Kenel, Albin; Kaufmann, Walter (2025)
Structural Concrete
Strip loading of reinforced concrete is a common problem in engineering practice. The concrete compressive strength at the load introduction can be several times higher than the uniaxial concrete compressive strength, which can be attributed to geometrical and passive confining stresses caused by load dispersion and reinforcement, respectively. However, current design provisions do not allow combining these two confining effects, leading to overly conservative designs. Furthermore, experimental data on strip-loaded concrete members with pronounced passive confinement and moderate load concentration ratios, common, for example, in longitudinal joints of segmental tunnel linings, are scarce. This article presents different models for strip loading as well as an experimental campaign of 15 concrete blocks subjected to strip loading. The test parameters included (i) the reinforcement ratio and type, (ii) the width and position of the loaded area, and (iii) the concrete strength. Digital image correlation was applied to determine the crack widths. The experimental results give insight into the mechanical behavior and were used to compare the different model approaches for strip loading, including the recently published Dual-Wedge stress field and a simplification of the same. The models are presented in detail and adjusted for strip loading with moderate load concentration ratios where required. Furthermore, it is discussed how passive confinement can be accounted for with regular and irregular reinforcement spacings. The bearing capacity predictions of models accounting for the combined effect of load dispersion and passive confinement are in good agreement with the tests and allow for an efficient and safe design, whereas the simplification stands out for its ease of use and lowest coefficient of variation.
Determining the strength of 3D printed concrete with the modified slant shear test
Licciardello, Lucia; Soto, Alejandro Giraldo; Kaufmann, Walter; et al. (2025)
Structural Concrete
This study deals with the assessment of the load-bearing capacity of layered 3D printed concrete (3DPC) using a modified slant shear test (MSST) to define a Mohr's envelope. To this aim, the established modified Coulomb yield condition of the concrete is complemented by a Coulomb failure criterion of the layer joints (cold or dry joints). Due to the anisotropy inherent to 3DPC elements, caused by the interlayer of printed concrete, the test setups commonly used for conventional concrete are inadequate to assess the load-bearing capacity of 3DPC. A total of 45 3DPC specimens with different inclinations of the concrete printed layers, including a cold joint with 30 min time gap were tested using the MSST. The results showed a high strength of the interfaces with two different failure modes, depending on the inclination of the layers: (i) concrete matrix failure and (ii) layer interface failure, where the latter governed for layer inclinations of 60 degrees and 75 degrees with respect to the horizontal plane. The test campaign confirms that the proposed MSST is a highly practical test method that enables determining the parameters governing a Mohr's envelope of layered 3DPC and can thus be used to reliably characterize its load-bearing capacity.
Shrinkage cracking in restrained FRC members containing conventional reinforcement
Tang, Porsiem; Amin, Ali; Gilbert, R. Ian; et al. (2024)
Structural Concrete
This paper presents a rational approach for describing the cracking behavior of fully- and partially restrained fiber reinforced concrete members co-reinforced with conventional reinforcement subjected to an axial force imparted by shrinkage. The proposed analytical model extends the approach developed by Gilbert for fully restrained reinforced concrete members to account for the post-cracking strength offered by the fibers at each of the cracks as the concrete ages and dries. The effects of partial end-restraint are also studied to gain a clearer understanding of the mechanism of direct tension cracking caused by restrained shrinkage and the factors affecting it.
Rotational springs for localized crack modeling in structural concrete
Näsbom, Andreas; Thoma, Karel; Weber, Marius; et al. (2023)
Structural Concrete
This paper presents a mechanical and numerical approach to model localized deformations occurring in concrete structures with less than minimum reinforcement. The presented models are based on, and validated against largescale experiments conducted in a previous study to investigate the behavior of a less than minimally reinforced slab in the framework of the structural safety assessment of an existing cut and cover tunnel. The model deploys rotational springs embedded in a nonlinear finite element analysis to capture deformations concentrating in an inclined crack. An iterative solution procedure ensures equilibrium and compatibility in the inclined crack section by adjusting the spring stiffnesses, which depend on the acting loads. A case study shows that the localized crack has a minor effect on the global structural behavior. In contrast, localization strongly influences deformations and stress resultants in the crack, and compressive membrane action significantly impacts the load–deformation behavior of the localized crack, which is relevant particularly for the shear strength.
Load‐deformation behavior of locally corroded reinforced concrete retaining wall segments: Experimental results
Häfliger, Severin; Kaufmann, Walter (2023)
Structural Concrete
Local reinforcement corrosion damage reduces the load-bearing capacity of reinforced concrete structures and, even more severely, their deformation capacity. This problem is of particular concern for cantilever retaining walls, whose loading is dominated by earth pressure and hence, depends on the wall deformations. With a limited deformation capacity at the ultimate limit state due to the locally corroded reinforcement, the earth pressure may not drop to its reduced value typically assumed in design, and simultaneously, the structural resistance may be severely impaired by the cross-section loss. Load redistributions are impeded since retaining walls are statically determined vertically and typically segmented longitudinally. This increases the risk that affected structures collapse, exhibiting a brittle failure. The situation is aggravated by the fact that the wall deformations prior to failure are too small to be detected by conventional monitoring, as indicated by a previous study. To improve the basis for quantifying the related risks and the magnitude of prefailure deflections, this study investigates the load-deformation behavior of cantilever retaining walls affected by local pitting corrosion, focusing on (i) the influence of the corrosion pit distribution among different reinforcing bars on the load-bearing and deformation capacity and (ii) the interdependence of corrosion, reduced deformation capacity and deformation-dependent loading. To this end, eight large-scale experiments on retaining wall segments were conducted in the Large Universal Shell Element Tester (LUSET), simulating the lower part of a 4.65-m-tall cantilever retaining wall. Five specimens contained initial damage (pitting corrosion simulated by a spherical mill). In the remaining three specimens, artificial corrosion damage was induced during the experiments. For two of the latter specimens, the loading was adapted in real-time control depending on their deformation to simulate the decreasing earth pressure. These are the first large-scale hybrid tests in the field of corrosion research to our knowledge. The experiments confirmed that the ultimate load and the corresponding deformation strongly differ depending on the corrosion pit distribution, even among specimens with equal mean cross-section loss. Furthermore, it was found that the deformation increase due to corrosion damage depends on the loading and, hence, on the compaction of the backfill. The observed deformation increase ranged between 0.8 and 1.4 mm per meter height at 40% crosssection loss, with loose soil causing a larger deformation increase. The load transfer between the damaged and undamaged reinforcing bars was found to take place in the first two crack elements above the construction joint. Local bending moments occurred in the reinforcing bars in the vicinity of the corrosion pits due to the shift of the center of gravity of the bar at the pit. Fiber optic strain sensing allowed visualizing the bending moment decrease in the embedded part of the damaged bars as a consequence of a lateral bearing pressure.
Assessment of parameters influencing the behavior of statically indeterminate reinforced concrete slabs and beams under fire conditions
Bischof, Patrick; Morf, Urias; Kaufmann, Walter (2023)
Structural Concrete
This paper investigates the effect of material properties, boundary conditions, and related modeling and design uncertainties on the fire behavior of statically indeterminate reinforced concrete beams and slabs by means of a parametric study based on a comprehensive model. This model uses material properties specified by EN 1992-1-2, complemented by considerations concerning the biaxial compressive strength of concrete, strain hardening and limitations of the ultimate strain of reinforcement, as well as tension stiffening. The parametric study identifies and explains the most influential parameters governing the fire behavior of statically indeterminate reinforced concrete beams and slabs. The implementation and generalization of these parameters are evaluated and compared to current design rules in EN 1992-1-2 derived from the evaluation of experimental testing and real fire cases. Overall, the detailing rules given in EN 1992-1-2 are found to be reasonably safe, and they can be easily used for practical applications. Furthermore, the results of the study indicate that model predictions for the studied statically indeterminate systems are subject to considerable uncertainty because (i) information on the used material is possibly incomplete and (ii) the models given in design codes do not (or only insufficiently) cover all relevant aspects of the thermo-mechanical behavior. Specifically, the concrete aggregate type with its corresponding thermal expansion, the strain hardening properties of the reinforcement and tension stiffening with its detrimental effect on the ductility of the tension chord may affect the rating across several standard fire resistance times of statically indeterminate reinforced concrete members subjected to bending.

Publications 1 - 10 of 32

Journal: Structural Concrete

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